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Radar systemRadar system description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060055585, Radar system. Brief Patent Description - Full Patent Description - Patent Application Claims
CLAIM OF PRIORITY [0001] The present invention claims priority from Japanese application JP 2004-164097 filed on Jun. 2, 2004, the content of which is hereby incorporated by reference into this application. FIELD OF THE INVENTION [0002] The present invention relates to a radar system for detecting distance up to an object, relative velocity and direction by receiving the reflected wave of the electromagnetic wave radiated to the object and returned therefrom after reflection and particularly to a radar system using the electromagnetic waves of a couple of signals having the frequencies which are a little different or the frequency-modulated signals. BACKGROUND OF THE INVENTION [0003] The radar system for detecting distance, relative velocity, and direction of an object by radiating the electromagnetic wave and receiving the wave returning from the object after reflection has been widely used. Typical radar system of this type includes a trespasser monitoring radar which issues an alarm by detecting a person or an animal invading into the monitoring area and a vehicle radar used for an adaptive cruise control system for vehicle. [0004] In the radar system, a method for detecting the distance up to an object is classified into a two-frequency CW (continuous wave) system using two continuous wave signals having the frequencies which are a little different and an FMCW (Frequency Modulation Continuous Wave) system using the continuous wave signal which is changed continuously in the frequency. [0005] As an example of the two-frequency CW system, an example is disclosed in the Patent Document 1, in which different two couples of two-frequencies are used respectively for long distance measurement and short distance measurement and an error detection in the short distance measurement is prevented by roughly detecting an obstacle located in the outside of the detection area with the two-frequency for long distance measurement. Moreover, another example of the two-frequency CW system, an example is disclosed in the Patent Document 2, in which deterioration in accuracy generated through multiple reflections between an antenna and the object by obtaining the reference phase difference at least from one period of the phase difference signal of the receiving signal. [0006] [Patent Document 1] JP-A No. 166443/1996 [0007] [Patent Document 2] JP-A No. 39009/1998 SUMMARY OF THE INVENTION [0008] In order to describe the problems to be solved in the present invention, the principle of the two-frequency CW system will be described first. [0009] FIG. 14 illustrates an example of the basic structure of a radar system of the two-frequency CW system. The two-frequency CW system transmits two signal frequencies which are a little different with each other as the transmitting signals. In the example of FIG. 14, these transmitting signals are generated with a voltage-controlled oscillator 1 which is controlled with a frequency control voltage from a control voltage generator (CONT) 2. Here, the transmitting signals S.sub.1TX and S.sub.2TX are expressed by the formulae (1) and (2). S.sub.1TX=A.sub.TX sin(2.pi.f.sub.1t) (1) S.sub.2TX=A.sub.TX sin(2.pi.f.sub.2t) (2) [0010] Here, f1, f2 are frequencies of the transmitting signals, and A.sub.TX is a signal amplitude of the transmitting signals. [0011] This signal is radiated from the antenna 4. The receiving signals S.sub.1RX, S.sub.2RX for the transmitting signals S.sub.1TX, S.sub.2TX are expressed by the formulae (3) and (4). S 1 .times. RX = A RX .times. sin .function. [ 2 .times. .pi. .function. ( f 1 + f d1 ) .times. t - 4 .times. .pi. .times. .times. f 1 .times. R C ] ( 3 ) S 2 .times. RX = A RX .times. sin .function. [ 2 .times. .pi. .function. ( f 2 + f d2 ) .times. t - 4 .times. .pi. .times. .times. f 2 .times. R C ] ( 4 ) [0012] A.sub.RX is a signal amplitude of the receiving signals, f.sub.d1, f.sub.d2 are Doppler shift generated by the relative velocity between the radar system and an object, R is distance between the radar system and an object, and c is the velocity of light. [0013] Here, when .DELTA.f=f1-f2<<f1, f2, following relationship can be obtained. f.sub.d1.apprxeq.f.sub.d2=f.sub.d [0014] This signal is applied to a mixer 3 to generate the low frequency signals S.sub.1IF and S.sub.2IF having only the Doppler frequency element expressed by the formulae (5) and (6). S 1 .times. IF = A IF .times. sin .function. [ 2 .times. .pi. .times. .times. f d .times. t - 4 .times. .pi. .times. .times. f 1 .times. R C ] ( 5 ) S 2 .times. IF = A IF .times. sin .function. [ 2 .times. .pi. .times. .times. f d .times. t - 4 .times. .pi. .times. .times. f 2 .times. R C ] ( 6 ) [0015] Here, A.sub.IF is a signal amplitude of the low frequency signal. This signal is subjected to the process such as the Fast Fourier Transfer with a signal processing circuit (PRC) 6 to calculate a phase difference and a Doppler frequency. [0016] Here, the distance R up to the object can be expressed as the formula (8) from the phases of both signals expressed by the formula (7) and the distance is calculated with the signal processing circuit 6. .DELTA..PHI. = 4 .times. .pi. .function. ( f 2 - f 1 ) .times. R C = 4 .times. .pi..DELTA. .times. .times. f .times. R C ( 7 ) R = c .times. .times. .DELTA..PHI. 4 .times. .pi..DELTA. .times. .times. f ( 8 ) [0017] As described above, since distance is measured from phase difference in the two-frequency CW system, this system has a merit that it is no longer required to sweep the frequency for wider range and therefore it is suitable for higher accuracy and resolution in measurement of distance. Moreover, as expressed by the formula (8), since phase difference .DELTA..phi. is proportional to frequency difference .DELTA.f, amount of variation of phase difference .DELTA..phi. for distance R, namely gradient of .DELTA..phi. for R becomes large and resolution of distance can be improved. [0018] In the formula (8), the range for uniquely obtaining distance R (called "effective distance" in this specification) R.sub.max is defined with the range where phase difference .DELTA..phi. is equal to or less than 180 degrees. However, when an object existing in the position where .DELTA..phi. is equal to or larger than 180 degrees is received, the signal of this object is calculated under the assumption that the object is located in the position within the effective distance R.sub.max which should not actually be considered. As a result, the radar system generates "ghost" because it erroneously judges that the object exists in the position where nothing is actually located. [0019] In order to prevent generation of such ghost, .DELTA.f must be set so that the effective distance R.sub.max becomes larger than the detectable limit distance which is determined with the radar performance such as radiation power of transmitter and sensitivity of receiver which are forming the radar system. In this case, when R.sub.max is larger, .DELTA.f becomes small and distance resolution is lowered. [0020] On the contrary, if it is attempted to make larger .DELTA.f in order to raise distance resolution, R.sub.max becomes small. As described above, the trade-off relationship exists between .DELTA.f and R.sub.max. [0021] In the case of a vehicle radar system, an object to be detected is an ordinary vehicle or a large duty truck. These are substances which reflect particularly a large amount of electromagnetic wave among those existing in the environment for use and show a little difference in amount of reflection of electromagnetic wave in accordance with types of vehicle. Therefore, the detectable limit distance and effective distance R.sub.max determined by the performance of radar system can be set to almost equal values. [0022] Meanwhile in the case of the radar system for monitoring a trespasser for detecting a person who is intruding into the detectable range, a human body is mainly selected as an object to be detected. In general, a human body shows a very small amount of reflection of electromagnetic wave such as about 1/10 to 1/1000 in comparison with a vehicle. It is assumed here that the detection range is set to several tens meters and performance of radar system is set to the range for detecting a human body located within the preset range. In this case, since the reflected signal from a substance which shows a large amount of reflection of electromagnetic wave such as a vehicle existing in the area separated by several hundreds meters becomes almost equal to the reflected signal from a person within the range of several tens meters, the vehicle existing in the distant place is erroneously detected as a ghost as if it were within the detection range. In order to prevent generation of such ghost, the effective distance R.sub.max must be set to several hundreds meters. As an example, the radar system having the performance to detect up to a human body within the distance of 50 meters can spread the detection range by about 400 meters for vehicles. Accordingly, when it is assumed that vehicles run in the detection range, the effective distance R.sub.max must be set to about 400 meters in order to prevent generation of ghost. In this case, .DELTA.f becomes small in comparison with that when the effective distance R.sub.max is set to 50 meters. Continue reading about Radar system... Full patent description for Radar system Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Radar system patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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